Corrosion is the natural process by which metals, due to the environment to which they are exposed, revert to more chemically stable oxides. The more chemically stable oxides are more electrically resistive and do not conduct electrical current as well as the metal. By periodically measuring the change in resistance of a thin strip of metal exposed to the environment, the accumulation of corrosion damage over time, and the rate of corrosion, can be measured; as the metal strip becomes thinner, its resistance increases. The rate of corrosion is dependent on material composition and surface-specific conditions, as well as the level of corrosion protection applied to the metal. An example of a relevant surface specific condition is found in ductile iron pipe where the annealing oxide formed on the pipe surfaces during manufacture of the pipe reduces the rate at which such a pipe corrodes.
Cathodic protection (CP) is a known technique for controlling the corrosion of a metal surface. In this technique, the metal surface is utilized as a cathode of an electrochemical cell. Using CP, metal structures are better protected from corrosion. CP systems are commonly used to protect water and fuel pipelines, storage tanks, steel pier piles, ships, offshore oil platforms, and onshore oil well casings.
A sensor may be used to measure (or monitor) the effectiveness of a CP system when the sensor is applied, for example, to an oil or gas pipeline, to a potable water pipeline, or to a sewer line. Oil and gas pipelines, oil storage tanks, and other similar structures may be made of steel. Water pipelines may be made of ductile iron (DI). DI pipes have non-round cross-sections and have irregularly shaped exterior surfaces defined by the annealing oxide formed as such pipes are manufactured.
Electrical resistance (ER) sensors are known in the art; they are approximately 3 inches long and 1.2 inches wide and have thicknesses in the range of 0.5 to 0.75 inches. A sensor of this type often, if not usually, is connected to a measuring instrument or device via a connecting cable, having a diameter of 0.4 inches, for example. The sensor is secured, as by bonding directly to a DI pipe, and the pipe and the sensor are wrapped or encased with polyethylene or are covered by some other suitable protective material such as a mortar, for the purpose of protecting the pipe (a common structure of interest) from ambient conditions, notably soil.
However, the size of the sensor (as well as the diameter of the connecting cable) can cause the polyethylene wrap to “tent” and thereby provide a path through which groundwater or moisture can penetrate the wrap and make contact with the pipe, thereby defeating the purpose of the wrapping or encasement around the pipeline. In addition, the size of the sensor can limit its applicability to mortar coated pipe joints and in other situations requiring a lower degree of physical interference. A need exists for an improved ER sensor which is of significantly reduced cross-sectional area, i.e., lower profile, and which has a connector cable of reduced cross-sectional area, to enable better isolation of an instrumented pipeline, e.g., from ambient conditions and to enable sensor use in places where existing sensors cannot be used effectively.
While corrosion of pipelines and other structures of interest (SOIs) has long been known to involve the effects of direct currents flowing in the structure, there is a growing concern that alternating currents present in such structures can produce corrosion. Alternating currents can be induced in support towers for electrical power transmission lines and in other structures, such as buried pipelines. A need exists for corrosion sensor arrangements which permit and enable the detection and measurement of corrosion due to the effects of either or both of direct and alternating currents.
Further, existing ER corrosion sensors have inadequate sensitivity for short-term use, such as in the making of measurements for determining proper CP to be applied to a SOI. A need exists for an ER corrosion sensor having increased sensitivity.